This invention relates to a surface acoustic wave device, and particularly to a surface acoustic wave device used as a band-pass filter and having a circuit construction comprising a plurality of surface acoustic wave (SAW) resonator filters connected in the form of a ladder.
High-frequency filters of mobile communication devices such as portable telephones in which SAW resonator filters are used have been proposed. As a surface acoustic wave device used in this kind of application, a surface acoustic wave device having the connection structure shown in FIG. 1 has been proposed.
In this surface acoustic wave device, a three-electrode vertically connected double mode SAW resonator filter 1 is disposed between an input terminal IN and an output terminal OUT. This vertically connected double mode SAW resonator filter 1 has three inter digital transducers (hereinafter referred to as IDTs) 2 to 4 arranged along a SAW propagation direction. Reflectors 5 and 6 are disposed at the surface wave propagation direction ends of the arrangement of IDTs 2 to 4.
One comb electrode 2a, 4a of each of the IDTs 2 and 4 among the IDTs 2 to 4 is connected to the input terminal IN by way of a first one-port SAW resonator filter 7. One comb electrode 3a of the IDT 3 is connected to the output terminal OUT. The other comb electrodes 2b, 3b and 4b of the IDTs 2 to 4 are connected to a ground potential.
Also, a second one-port SAW resonator filter 9 is connected between the ground potential and a connection point 8 between the comb electrode 3a and the output terminal OUT.
Accordingly, in the surface acoustic wave device shown in FIG. 1, a vertically connected double mode SAW resonator filter 1 is connected between input and output terminals and a first one-port SAW resonator filter 7 is connected in series with the input side of this vertically connected double mode SAW resonator filter 1. Also, a second one-port SAW resonator filter 9 is connected between a reference potential and a point between the input and output terminals. Thus, the surface acoustic wave device shown in FIG. 1 comprises a filter circuit having two series resonators and one parallel resonator.
Here, the resonance frequency of the first one-port SAW resonator filter 7 is within the pass band of the vertically connected double mode SAW resonator filter 1 and the antiresonance frequency of the second one-port SAW resonator filter 9 is within the pass band of the vertically connected double mode SAW resonator filter 1.
In the surface acoustic wave device described above, because the first one-port SAW resonator filter 7 is connected to the outer IDTs 2 and 4 of the vertically connected double mode SAW resonator filter 1 and also has the above-mentioned resonance characteristic, the VSWR in the outer IDTs 2 and 4 of the vertically connected double mode SAW resonator filter 1 is reduced and also the amount of attenuation outside the pass band and particularly in the blocking region on the high-frequency side is increased.
Also, because the second one-port SAW resonator filter 9 has the above-mentioned resonance characteristic, the VSWR in the central IDT 3 of the vertically connected double mode SAW resonator filter 1 is reduced and also the amount of attenuation outside the pass band and particularly in the blocking region on the low-frequency side is increased.
Therefore, it is possible to reduce losses in the pass band, reduce the VSWR in the pass band and increase the amount of attenuation in the blocking region.
However, when the surface acoustic wave device shown in FIG. 1 is used, for example, in an antenna of a portable telephone or the like, in the blocking region of the receiving side filter a large electrical power signal from the transmission side is impressed. Consequently, with the surface acoustic wave device construction described above, there has been a problem that when a large power of for example about 2 W is impressed on the device it instantaneously breaks down.
Also, when the surface acoustic wave device described above is used as a receiving side filter of an antenna of a portable telephone or the like, normally, the surface acoustic wave device is connected using a strip line or the like to a transmission side filter made using for example dielectric resonators or a surface acoustic wave device. That is, the surface acoustic wave device described above and a transmission side filter are connected using a strip line or the like so that the impedance in the blocking region is open.
However, in this case, to suppress transmission side losses, it is highly desirable that the reflection coefficient of the receiving side filter in the transmission side pass band be large. But when the surface acoustic wave device described above is used as the receiving side filter, the reflection coefficient thereof in the transmission side pass band has not always been sufficient.